There is an ongoing application-explosion involving the manipulation and management of microscopic quantities of fluids for useful purposes. No application serves as a better example than the numerous permutations of inkjet-printers for commercial and personal printing applications that employ inks or marking materials. A multitude of methods for creating droplets and transferring them to substrates such as paper in desired patterns are known and many others are under development. The known methods include thermal-jetting drop-on-demand, piezo-jetting drop-on-demand, and pressurized continuous inkjets with electrical droplet steering. New methods under development and seen in the patent literature include ballistic aerosol printing and ballistic aerosol printing with gas flow droplet-deflection. There are many more not mentioned.
For the purposes of the invention herein, we define a “microfluidic device” as any device or component that utilizes, implements or supports the storage, management, arrangement, manipulation, analysis, processing or distribution of microscopic quantities of at least one working material. Thus, this clearly includes any droplet or particulate emitter used for any purpose as well as substrates or “labs-on-a-chip” upon or within which microfluidic quantities of material are stored, arrayed, combined, compared, analyzed, processed or otherwise manipulated. Examples would include all inkjets and combinatorial bioarrays made using “inks” comprising biological fluids as well as fluidic-incorporating “labs-on-a-chip” for clinical testing or environmental chemical sensing. By “fluid” we mean any flowable (or diffusible) material, mixture, suspension, solid, emulsion, solution, vapor, gas, liquid, slurry, fluidized media such as suspended cellular material, gel, cream, wax, oil, hydrocarbon, paste or solution. In short, we define a fluid as anything that can be moved or moves along at least one path, whether by net mass-transport, liquid flow, gas flow or even atomic or molecular flow as moving diffusing concentration-gradients. Such flow may be over macroscopic or microscopic distances or across a permeable or other membrane in the device.
Despite all of the investment in microfluidic devices, there are still some fundamental issues and challenges that have not been overcome to anyone's satisfaction. Solutions to these issues would provide further reduced costs, further reliability improvements, further inkjet image-quality improvements and better performing biochips. Some of these unsolved issues addressable by the invention herein are as follows:
Ink or Other Media Fouling and Clogging of Printheads and Nearby Printer Components.
Tiny nozzles and orifices tend to clog if they dry out or if they are contaminated during nozzle self-servicing steps involving wipers or scrapers. The trends toward jetted pigment-based inks and biological fluids are only making matters worse.
Ink, Media and Debris-Contamination and Wetting of the Printhead Orifice Face.
If the face of the printhead becomes fouled, then the orifice ink wets out onto the surface and causes misfires and unwanted deflection of droplets.
Shutdown, Startup and Priming of Printheads.
As printheads incorporate more and more fine orifices and channels, lumens, and conduits, the opportunity for the printhead to incorporate (e.g., grow) bubbles or other blockages during long standby periods increases. In particular, outgassing of ink and ingress of atmospheric gas can cause blocking bubbles in fine channels. Even inks with water-retention features such as glycol or hydrophilic constituents can eventually dry out or at least uncontrollably thicken at the ambient interface.
Use of Ink Dryer Modules.
There is a lot of developmental activity in the area of methods to dry ink quickly so that printed paper, for example, can be stacked soon after printing without smudging. Such suggested techniques involve everything from microwave drying to blown gases and infrared radiation. As one can easily discern, uncontrolled and unintended heating or convective drying of the printheads and their orifices could greatly worsen many of the above listed challenges.
Cartridge Life and Reliability Issues.
The management of dissolved gases in inks is becoming a major issue both for onaxis and off-axis ink tank strategies. Such air incorporation in an uncontrolled manner can lead to bubbles and unpredictable emission-bubble formation. It can also cause unpredictable cavitation in piezo-fired printheads and misfiring of thermal-bubblejets.
Customers have also complained loudly about perceived cartridge lifetime issues and perceived wasted-ink issues. The invention herein also offers a new method of ink provision that can avoid some of these difficulties, perceived or otherwise.
We emphasize the inkjet printer applications by way of example, but the reader will realize that the invention is equally applicable to microfluidic-based labs-on-a-chip wherein microparticulates, liquids and gases are processed and one has similar issues of shelf-life, clogging, material storage, and useful-life.
By inkjet printing we include all droplet or particulate microemission applications, whether continuous or drop-on-demand, regardless of the marking material involved. The marking material could be ink or could be microdroplets of biofluids being placed on or in a combinatorial bioarray, for example. By lab-on-a-chip we include any microfluidic component having miniature or microscopic conduits, reservoirs, valves or manifolds. This would include, for example, blood analysis labs-on-a-chip, urine analysis labs-on-a-chip, DNA analysis labs-on-a-chip, and chemical sensor arrays on-a-chip.
Thus, the present invention should be seen as offering generic improvements to the field of microfluidics in general, with microfluidics being defined as the manipulation, storage and/or processing of minute quantities of materials, as stated earlier.
The appendix contains a set of reference patents useful for understanding the applications of the invention herein. These patent references in no way lead to any embodiment of the invention but they do help the reader appreciate the seriousness of some of the challenges that we wish to solve with our inventive embodiments, and they demonstrate attempts at solving some of these issue to date.